A large, x-ray selected sample of extragalactic objects found "serendipitously" in Einstein Observatory Imaging Proportional Counter exposures at high galactic latitude will be optically identified and further investigated. The primary objectives of this study will be to measure the x-ray luminosity function and cosmological evolution of Active Galactic Nuclei (AGNs), including quasi-stellar objects (QSOs), quasars and Seyfert galaxies, BL Lacertae objects and clusters of galaxies. For the BL Lacertae objects and clusters, x-ray selection offers the only objective and complete sample approach toward studying these objects and hence finds AGNs not easily found by any other means (thus complementing radio and optical selection techniques). Once obtained, these basic data will be used to achieve the following major goals: (1). To measure the x-ray luminosity function and cosmological evolution of AGNs, BL Lacs and clusters of galaxies; (2). To integrate these luminosity functions to obtain the discrete source contribution to the x-ray background to a higher accuracy than previously; (3). To compare the cosmological evolution of BL Lacs with the evolution of AGNs to determine the relationship between BL Lacs and quasars; (4). To measure the cosmological evolution of x-ray emission in clusters of galaxies, to z equal to a redshift of 0.3, to compare with models of the evolution of cluster potential wells with cosmic time; (5). To accurately measure the absolute surface density of extragalactic x-ray sources and their observed x-ray, radio and optical characteristics.
